The external gravitational field produced by a rigid body of uniform density but irregular shape is formulated in terms of spherical harmonics. The formalism is applied to the Martian satellite Phobos. Based on a 3-dimensional shape model of Phobos by Duxbury, the gravitational coefficients up to degree and order 4 for a homogeneous Phobos are computed. In particular, we find J2=0.105. The in-plane libration amplitude of a homogeneous Phobos is predicted to be 0.97¿, within the rather large uncertainty of the observed value of 0.78¿0.4¿. ¿ American Geophysical Union 1989